Conventional metal halide discharge light sources typically comprise a fused silica tube with two electrodes, a rare gas for starting, a charge of mercury, and a fill comprising one or more metal halide salts, generally the iodides.
In their operation, a starting voltage of about 300 V is applied across the electrode gap causing the contents of the arc tube to vaporize, resulting in a high temperature, high pressure, wall stabilized arc in a gas, consisting principally of mercury vapor, ionized metal atoms and iodine molecules.
The output spectrum (i.e., the color of the discharge) of metal halide discharge lamps consists predominantly of the spectrum of the added metal halides. Color output for such lamps is tailored by varying the types of metal halides added to the arc tube. See for example, Waymouth, "Electric Discharge Lamps," Chapter 8, MIT Press, (1971).
The present invention represents a radical departure from the preexisting technology, namely the discovery of a metal halide arc lamp that does not employ mercury, the source cell of which can be formed from conventional glass, and which operates at near ambient temperature by means of a short duration, high pulse current.